Human leukocyte antigen (HLA) is known to include multiple HLA types, such as HLA-A, HLA-B, HLA-C, HLA-DP, HLA-DQ, HLA-DR, and HLA-MICA. An HLA allele is designated with a four or more digit number by the WHO HLA Nomenclature Committee. The principle of the nomenclature is that the first two digits correspond to the serotypes; the third and fourth digits distinguish the alleles of different amino acid sequences (subtypes); and the fifth digit distinguishes the alleles of different base sequences but encoding the same amino acid sequence. Typing of these alleles has been conventionally conducted at the serological level. Although this serological method does not require special sample processing, and enables easy typing using antigen-antibody reaction, the serotypes are the roughest classification corresponding to the first two digits of the numbers according to the nomenclature described above.
Many of other commercially available kits of the type associated with genomic extraction do not have enough accuracy to identify each allele individually. It is the current state that such a kit distinguishes multiple alleles as a group. Moreover, even a kit based on the SBT (Sequencing Based Typing) method, which enables the most detailed polymorphic analysis, often fails to solve the problem of ambiguity by one analysis since most samples are heterozygotes requiring reexamination. Such problematic alleles are listed collectively in http://www.ihwg.org/protocols/sbt/ambiguities2.pdf by the International Histocompatibility Working Group (IHWG).